Cross-talk suppression



Jan.- 24, 1956 J. E. BoUGHTwOoD 2,732,431

CROSS-TALK SUPPRESSION Filed April 17, 1951 5 Sheets-Sheet l yJim 24,1956 .1. E. BoUGHTwooD 2,732,431

CROSS-TALK SUPPRESSION Filed April 1'7, 1951 5 Sheets-Sheet 2 FIG. 2 J

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T1 1E- 1. E. BouGHTwooD ATTORNEY Jan 24, 1956 J. E. BoUGHTwooD 2,732,431

CROSS-TALK SUPPRESSION Filed April 17, 1951 5 SheebS-Shee'I 3 FIC-3.3

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J. E. BOUGHTWOOD ATTORNEY Jan. 24, 1956 J. E. BoUGHTwOoD CROSS-TALKSUPPRESSION 5 Sheets-Sheet 4 Filed April 17. 1951 TIME- P .INVEN TOR.

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l ATTORNEY Jan. 24, 1956 Filed April 17, 1951 R EC.

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5 Sheets-Sheet 5 l I Yl i C I D DETECTOR AMPLIFIER I I sTAGEs C a. D-II- g I I l I ll- I 1 l I l E I l F I DETECTOR AMPLIFIER I M I STAGES EaF l I I I- l I I I I I l G I I l DETECTOR AMPLIFIER I I STAGES G H I lI -II- l INVENTOR. 65/ J. E. BouCI-ITwooD ATTORNEY United States PatentO CROSS-TALK SUPPRESSION Jann E. Boughtwood, Halesite, N. Y., assignorto The Western Union Telegraph Company, New York, N. Y., a corporationof New York Application April 17, 1951, Serial No. 221,470

' 5 Claims. (Cl. 179-15) The present invention relates to pulsemodulation systems and more particularly to suppressing cross-talkbetween the channels thereof.' K

In pulse modulation systems wherein the transmitting time is dividedinto discrete intervals divided among the differentv intelligencechannels, it is important that the signals representing intelligencesamples do not overlap, or if they do overlap that such overlapping issuppressed before the intelligence samples are applied to the respectivereceiving channels. If such overlapping is not suppressed, intelligenceintended for one channel will appear' as interference in one or moreother channels. This interference, which is commonly termed cross-talk,may be sutiiciently strong to render the systemunusable orf at leastseriously to impair the operation thereof.

The chief cause of cross-talk in pulse modulation systems is distortionof the pulse wave shapes resulting from the non-linear phase-frequencyand amplitude-frequency characteristics of the transmitting medium andthe filters and other circuits of the transmitter and receiver.

Accordingly, it is an object of the invention to provide means forsuppressing cross-talk between channels of a pulse modulation system.

More particularly, it is an object of the invention to provide means forsuppressing cross-talk in a pulse modulation system resulting fromdistortion of the received pulse wave shapes.

Another object of the invention is to provide simple and easilyadjustable apparatus for suppressing cross-talk between channels of apulse modulation system.

Further objects of the invention will appear from the followingdescription.

In accordance with the invention, these objects are achieved byproviding means to shift the phase of a portion of the high frequencycomponents of the received pulses to correct the shape of the leadingedges of the received pulses, differentiating the received pulses tocorrect the shape of the trailing edges thereof, and by adding to thereceived pulses a modulation component of equal amplitude and in phaseopposition to the long term crosstalk present in the respectivereceiving channels.

The invention will now be described in greater detail with reference tothe appended drawing in which:

Fig. l illustrates the receiver of a pulse amplitude modulation system;

Figs. 2, 3 and 4 are diagrams illustrating the operation of the circuitof Fig. 1;

Fig. 5 illustrates the productionof long term crosstalk; and i Fig. 6shows a circuit for suppressing long term crosstallt.`

Referring now to the'drawing and more particularly to Eig; 1, thereceiver illustrated'therein is substantially the same asthe receiver ofthe pulse amplitude modulation system described in my copending U. S.patent application Serial No. 221,469, tiled concurrently herewith andnow Patent No. 2,672,517.

2,732,431 Patented Jan. 24, 1956 i 2 The receiver of Fig. l comprises aradio receiver 10, the output of which is applied to an amplifier andlow pass tilter 11, the output of which is, in turn, derived from acopending patent application.

potentiometer 12 and applied to the control grid 13 of an amplifier tube14 through a capacitor 15. The amplified output of tube 14 is applied tocontrol grid 16 of an electron discharge tube 17 through a couplingcapacitor 18. Anode 19 of tube 17 is coupled to a source of Vpositivepotential through a resistor 20, while cathode 21 is coupled to groundthrough a resistor 22. Resistors 20 and 22 are preferably equal inmagnitude so that the signal Avoltages developed thereacross will besubstantially equal and in phase opposition.

One end of a variable capacitor 23 is connected to anode 19. One end ofa variable resistor 24 is connected to cathode 21. The free ends ofcapacitor 23 and resistor 24 are connected together and to the controlgrid 25 of an electron discharge tube 26 through a coupling capacitor27.

Assuming a signal voltage having a given frequency npplied to controlgrid 16, the phase of the voltage applied to control grid 25 can beadjusted through substantially by varying the respective values ofcapacitor 23 and resistor 24 because the voltage applied to grid 25 isde termined by the relative proportions and phase relation ships of thevoltages at anode 19 and cathode 21 applied to grid 25. If the signalvoltage applied to control grid 16 comprises a plurality of frequencycomponents, the

'phase relationships between the components can be varied by varying therelative values of capacitor 23 and resistor 24 Anode 28 of tube 26 iscoupled to a source of positive potential through a resistor 29. Cathode30 of tube 26 is coupled to ground through two series connectedpotentiometers 31 and 32. The tappings of potentiometers 31 and 32 arecoupled to ground through variable resistor 33 and variable capacitor 34and variable resistor 35 and variable capacitor 36, respectively.Circuit elements 31 through 36 constitute a diierentiating network, thefunction of which will be described more fully hereinafter.

Anode 28 is coupled to ground through a variable capacitor 37 and to asynchronous gate 38 through coupling capacitor 39. Cathode 30 is coupledto frequency divider system FD, which is in turn coupled to synchronousgate 38 and to an electronic receiving distributor 40. The outputs ofreceiving distributor 40 are applied, respectively, todetector-amplifiers 41A through 41H. The outputs of detector-amplifiers41A through 41H are applied, respectively, to output terminals OAthrough OH through low pass iilters 42A through 42H, respectively. Theoutput of detector-amplifier 41A is also applied to a band pass filter43 which is coupled to distributor stepper 44. The output of stepper 44is applied to frequency dividers FD.

The circuits of the detector-amplifiers 41A through 41H will bedescribed more fully hereinafter in connection with Fig. 6 The circuitsand functions of synchronous gate 38, frequency divider system FD,receiving distributor 40, low-pass filters 42A through 42H, band passfilter 43 and stepper 44 are fully described in my copending patentapplication referred to hereinbefore.

In Fig. 2A there are illustrated idealized band pulses for applicationto amplifier tube 14 of Fig. 1. These pulses are substantially identicalto the sending Vdistributor output after slicing in a-gating circuit asdescribed in my said In Fig. 2 .the pulses for channels A through' E areindicated, channel C having modulationpresent thereinindicated `bycrosshatching. The pulses of Fig. 2a contain a large number of harmonic`components, so that transmission through circuits and media havingrestricted `band width will produce changes in wave shapes.

`might actually be present at'the output of amplifierand filter 11. Itwill be observed that the steep leading and trailing -edges'of thepulses ofl Fig. 2a have .beenlost. Furthermore, the `waves'corresponding to-thedifferent channels overlap,`which will tend toproduce cross-talk in the outputs of receiving distributorv 4t). Fig. 2cillus- `trates the wave shapes of Fig. 2b without modulation in channelC. yThe time intervals outlined by dashed lines A through E indicate theportions of the waves sliced bysynchronous gate '33. ln other words,only those Aportions of the pulseslying within the dashed lines will beapplied to receiving distributor A40. Use of a. gating circuitmateriallyI reduces cross-talk, but, 'as is evidentfrom Fig. 2c,overlapping during the gating intervals still occurs.

v Fig. 3aillustr-ates the wave-shape oflthe channel C pulse atthefoutput ofiilter 11 and the gating intervals B', C and D of channelsB, C and D, respectively. The leading and trailing edges of pulse Chaveslopingv Ashapes instead of the Vdesiredsharp rise and fall of Fig.

2a. The principal causes of this loss of desired shape of the leadingedge are the non-linear amplitude-frequency and phase-frequencycharacteristics ofthe circuits and media through which the pulse hasbeen transmitted.

In particular, restricted bandwidth and limited transient responsesproduce variations in amplitude and phase of the high frequency pulsecomponents with respect to the low frequency pulse components.

Adequate suppression of crosstalk does not require the received pulsesto be returned to their ideal shapes, but only that the pulse amplitudeof augiven channel be sharply reduced during the gating intervals ofother channels. This is accomplished with respect to the leading edgeofthe waves byvshifting the phase Vof a portion of the high frequencycomponents. Correction of the leading edge of the waves is accomplishedin the circuit of Fig. 1 by tube M17 and its associated phase shiftingcircuit.

The phase-f`requency characteristic of this phase shifting circuit isillustrated in Fig.A 3b, which' is a plot 0f phase vs. frequency. If thephases '.at anodev 19 and cathode 21 be assumed -as +90 and 90",respectively, at low frequencies capacitor 23 will appear as an opencircuit, so the voltageapplied to'grid 25 will' have a phase of -V9G".As thefrequency is increased,"the veffect of capacitor 23 becomes morepronounced, shifting the phase of the voltage `applied to grid 25. Athigh frequencies, capacitor' 23 acts substantially 'as a short circuit,so that the phase of the voltage.applied`to grid 25 will -besubstantially +90". WhenA pulse C is applied to the phase shiftingnetwork, the different frequency components thereof will 'be shiftedlinphase in accordance with the characteristic shown in Fig. 3b. Thesephase shifted components 'vary 'the' wave shapeiof pulse C, producing a4corrected waveshape at the leadingedge of the Wave.

The corrected pulse RC is shown in Fig. 3c. f From lFig. 3c it can beseen that lthe leading edge of wave RC does net appearv duringjtheVVgating interval: B. lThis phase shiftingcircuit vvill have a'negligibleeffect'onthe trailing edge of thewave.

Fig. 3d is a vector diagram of the voltages across the velements of thephase 'shifting circuit. The lphase of resultant voltage VR, which .isthe voltage-developed between the' junction of resistor 24 andcapacitor'i23 and groundjcan be varied through"'180 along locus curve Lby relative 'adjustment of resistor 24and capacitor v23,

Tilt/bichadjustment "va'ries voltages V24 and' V231tinmagnitudeuand`phase. vCapacitoi'v 23'lrenders theV circuit-freiquencyy sensitive, sothat the phaseshift producedV can be materially differentk at l highand.l lovv frequencies', jas indicatcdin Fig'.- 3b.

Corrected lwave RCstill overlaps. gatingffintervalf` D of channel D, asshown in Fig. 4a. This overlap .can be suppressed by differentiatingpulse RC to produce a pulse `TC," and combining pulse YTO with 'pulse'RC l to produce pulse RC. The pulses RC, TC and RC are illustrated inFigs. 4a, 4b and 4c, respectively. The differentiating 5 network forproducing the trailing edge correction comprises circuit elements 31through 36 in the cathode circuit of tube 26. This differentiatingcircuit also tends to teepen the leading. edge` of pulse R'C, asshown inFig.

"l" he trailing edge correcting circuit `will-not be aseffective if thetrailing c dge of pulse Cl exhibits-oscillatory excursions, asillustrated in Fig. 4d. However,.;these oscillatory excursions,as.illustrated in Fig. 4d, are compensated for in the circuit of Fig. 1 bypartially decoupling anodej'ZS with respect y.to'high frequencies bymeans of capacitor 37. Capacitor 37 effectively converts the wave ofFig. 4d into the Wave of Fig. 3a.

Cross-talk may also be produced through thesuppression of low frequencycomponents 'of the pulses. In Fig. 5a there is shown an ideal vpulsewithout modulation. Fig. 5b illustrates the pulse of'Fig. 5a afterhaving passed through a network which strongly attenuatesthe lowerfrequency components. It will be noted'that the pulse of Fig. 5bexhibits` a negative excursionV of relatively-short duration andrelatively large amplitude. 'If-suppression of low frequency componentsis reduced'throughfcareful design of the system, a pulse as shown' inFig.5c'inwhich the negative excursion iseXaggerated, will result. Thisnegative excursion is 'of relatively'small and substantially constantamplituden but long duration -relative 'to Athe .sampling4 intervals.Distortion Vof this type'yields long term cross-talk, that is,cross-talk appearing in'each of the channels With substantially 'equalamplitude.

Ain accordancev with the invention, long term cross-talk issuprressed'by adding a'm'odulation component of equal amplitude` and inp'haseopposition to lthe long' term crosstalk' in each of Athe receivingchannels. A suitable circuit is.shown" in Fig. 6, which illustrates'indetail thcdetectorarnplier stages 41A'through 41HY of Fig.I l.

' etectoramplifier stage 41A comprises a diodeerectiier 50, the cathode51 of which is coupled to ground through a resistor 52. The channel Asignal .pulses are .applied vtov cathode 51 through coupling capacitor53. Anode 54 ofv diode 'S0' is coupled to control grid 55Uof amplifiertube 56 through a coupling capacitor 57. Anode 54 is also coupled'toground throughtheparallel'combination of capacitorSSmand'resistor 59.Cathode 60 of tube 56 is coupied to ground-through-thelseriescombination of'biasing resistorsland 62. Control grid SS is coupled tothe junction of resistors 61and-62 'through a voltage'divider comprisingresistors Y63and 64. The junction of resistors63and 64-is coupledto-ground through a `variable capacitor 65. "-Anode'66 of tube 56r iscoupled to a source 4of positivel potentialwthrough a'resistor 67and' toterminal-A.

f The -otherA detector-amplifier stages Aare'identical with the v'stagedescribed'and hence will-inet lne-.described in detail.l lt is'important tonote,` however,fthatrxthepoint in each stagey correspondingto -thejunctionY ofresistors 63 andfttjis'coupledfto the sameterminalofcapacitor 65. Therefore,-a vportionof the outputsignal-ofJeach amplifier tube will appear across capacitor 65 and-Willbe .applied to the control grids of each of the amplifier'tubes.

The magnitude of the voltage'thus applied may'bead- 65 justedby varyingthe Valuenof capacitor 65. jThe'phase of this voltage is such as tosuppress the long termfcrosstalk in each ofthe stages. A.It is evidentthat lthe vmore nearly equal the cross-talk in each'stage'produc'ed bydistortion vinone channel, the more complete vvillfbelthe `suppressionlof long term cross-talk.

l' Whilethe invention has been describedin a'pa'rticular embodimentthereof and in a particular 'use,-' itvv is-not desired, that it belimited thereto, forobvious'Amodifications thereof will occur to1those'skilled inthe-arthwithout departing from the spirit and scope ofthe invention as set forth in the appended claims. n

What is claimed is:

l. A multi-channel pulse modulation system receiver, comprising phaseshifting means having an input circuit and an output circuit and beingarranged toshift the phase of frequencies lying within a'frst givenrange with respect to the phase of frequencies lying within a secondgiven range lower than said rst Vgiven range, means to apply receivedpulses having sloping leading and trailing edges and including frequencycomponents lying with in said rst and second given ranges to the inputcircuit of said phase shifting means thereby substantially to steepenthe leading edges of said received pulses, a differentiating networkcoupled to the output circuit of said phase shifting means, said networkincluding means to diiferentiate said pulses thereby to producecorrecting pulses, means to combine said correcting pulses and saidreceived pulses thereby substantially to steepen the trailing edges ofsaid received pulses, a plurality of receiving channels, means includingan electronic distributor arranged to commutate said receiving channels,means coupled to said combining means to apply said received pulses tosaid electronic distributor, means coupled to said receiving channels toderive therefrom modulation components proportional to long-termcross-talk present in said receiving channels, and means to apply saidmodulation components to each of said receiving channels in a sense tosuppress long-term cross-talk therein.

2. A multi-channel pulse modulation system receiver, comprising phaseshifting means having an input circuit and an output circuit and beingarranged t shift the phase of frequencies lying within a rst given rangewith respect to the phase of frequencies lying within a second givenrange lower than said rst given range, means to apply received pulseshaving slopingleading and trailing edges and including frequencycomponents lying within said iirst and second given ranges to the inputcircuit of said phase shifting means thereby substantially to steepenthe leading edges of said received pulses, capacitive means coupled tothe output circuit of said phase shifting means selectively to attenuatethe frequency components of said received pulses lying Within said firstgiven range, a dilerentiating network coupled to the output circuit ofsaid phase shifting means, said network including means to differentiatesaid pulses thereby to produce correcting pulses, means to combine saidcorrecting pulses and said received pulses thereby substantially tosteepen the trailing edges of said received pulses, a plurality ofreceiving channels, means including an electronic distributor arrangedto commutate said receiving channels, means coupled to said combiningmeans to apply said received pulses to said electronic distributor,means coupled to said receiving channels to derive therefrom modulationcomponents proportional to long-term crosstalk present in said receivingchannels, and means to apply said modulation components to each of saidreceiving channels in a sense to suppress long-term cross-talk there-1n.

3. In a- .multi-channel pulse modulation system receiver, comprisingcircuit means to receive signal pulses having a sloping trailing edge,an electron discharge tube having a n control grid coupled to saidcircuit means, means including a differentiating network connected inthe power circuit of said electron discharge tube, said networkincluding means to differentiate said signal pulses thereby to producecorrecting pulses, means to combine said correcting pulses and saidVreceived signal pulses thereby to steepen the trailing edges of saidreceived signal pulses, an electronic distributor, second circuit meanscoupled to the output of said electron discharge tube and to saidelectronic distributor, a plurality of receiving channels, meanscoupling said receiving channels to said electronic distributor.

4. The combination set forth in claim 3 including means coupled to saidreceiving channels to derive therefrom modulation componentsproportional to long-term cross-talk present in said receiving channels,and means to apply said modulation components to each of said re- Yceiving channels to suppress long-term cross-talk.

5. In a multi-channel pulse modulation system receiver, the combinationcomprising circuit means to receive signal pulses, a phase shiftingnetwork having an input circuit coupled to said circuit means and anoutput coupled to an electron discharge tube, a differentiating networkconnected in the cathode circuit of said electron discharge tube, aplurality of receiving channels, an electronic distributor arranged tocommutate said receiving channels, means coupled to the output circuitof said electron discharge tube to apply said received pulses to saidelectronic distributor, means coupled to said receiving channels toderive therefrom modulation components proportional to long-termcross-talk present in said receiving channels, and means to apply saidmodulation components to each of said receiving channels to suppresslong-term cross-talk.

References Cited in the file of this patent UNITED STATES PATENTS2,524,251 Bradley Oct. 3, 1950 2,553,572 Frum May 22, 1951 FOREIGNPATENTS 605,128 Great Britain July 16, 1948 g OTHER REFERENCES Theoryand Application of Electron Tubes, 1944, Reich, p. 357. y

